A known example of a load driving system for an electric vehicle is a motor driving system that includes a battery generating a specified DC voltage, a bilateral step-up chopper unit raising the DC voltage generated by the battery to become a power supply for driving a motor on the basis of the raised voltage, an inverter unit for driving the motor, and an electrolytic capacitor connected between the bilateral step-up chopper and the inverter unit for smoothing the DC voltage outputted from the bilateral step-up chopper unit (see JP-A-2001-275367, for example).
Moreover, a variable voltage system is known which raises a battery voltage using a step-up converter to supply the raised voltage to a main capacitor, and variably controls a system voltage outputted from the step-up converter according to the operating condition of the motor (see Hideto Hanada et al., “Motor Control and Boost Converter Control for Hybrid Vehicles,” TOYOTA Technical Review, Vol. 54, No. 1, August 2005, pp. 42-51 (in Japanese), for example). More specifically, in a motor system, system losses such as a motor loss (a copper loss+a core loss), an inverter loss (an on-loss+a switching loss), a step-up IGBT loss (an on-loss+a switching loss) and a step-up reactor loss (a copper loss+a core loss) are produced. A condition to minimize the system losses is to make the system voltage approximately equal to an induced voltage of the motor. Since the induced voltage varies depending on the operating conditions of the motor (a rotating speed and a torque), a variable control of the system voltage according to the operating condition of the motor can minimize the losses.
FIG. 15 is a block diagram showing the circuit configuration of the motor driving system described in the above JP-A-2001-275367 as an example of the circuit configuration of a related motor driving system. As shown in FIG. 15, the system has a configuration in which a bilateral step-up chopper unit 101, formed with a DC to DC converter circuit, is provided for raising a battery voltage Vb of a battery 100, a DC link circuit 102, including an electrolytic capacitor connected between a positive electrode side line Lp and a negative electrode side line Ln, is connected onto the output side of the bilateral step-up chopper unit 101, and an inverter unit 103 as a DC to AC converter circuit is further connected in parallel to the electrolytic capacitor in the DC link circuit 102. The inverter unit 103 supplies three-phase AC power outputted therefrom to a motor 104.
Here, the bilateral step-up chopper unit 101 is formed of an IGBT1 and an IGBT2 connected in series between the positive electrode side line Lp and the negative electrode side line Ln, and a reactor L inserted between the connection point of the IGBT1 and the IGBT2 and the positive electrode side of the battery 100. Moreover, the battery 100 forms a DC power supply of a high voltage by connecting several tens of units, each with several volts, in series.
In general, the rotating speed Nm of the motor 104 and the DC voltage Ed across the electrolytic capacitor in the DC link circuit 102 are controlled to have a relation as is shown in FIG. 16. Namely, with the DC voltage Ed made equal to the battery voltage Vb while the rotating speed of the motor 104 is between zero and a specified rotating speed N0, switching devices forming the inverter unit 103 are subjected to PWM control, by which the rotating speed of the motor 104 is controlled. Then, while the rotating speed of the motor 104 exceeds the specified rotating speed N0 to reach a specified rotating speed N1 larger than the specified rotating speed N0, the step-up rate of the battery voltage Vb is made gradually increased from 1 according to an increase in the rotating speed of the motor 104 by the bilateral step-up chopper unit 101. This, with the DC voltage Ed made gradually increased from the battery voltage Vb, makes switching devices in the inverter unit 103 subjected to PWM control, by which the rotating speed of the motor 104 is controlled.
Thereafter, with the rotating speed of the motor 104 exceeding the specified rotating speed N1 at which the DC voltage Ed reaches the maximum voltage EdMAX, the DC voltage Ed is made fixed at the maximum voltage EdMAX and, for a further increase in the rotating speed of the motor 104, the inverter unit 103 is made to shift its control from the PWM control to one-pulse control or made to carry out field weakening control.
However, in the above examples of related load driving systems, the minimum DC input voltage inputted to the inverter unit 103 to be the DC to AC converter circuit becomes the battery voltage Vb. From this, when the motor 104 is operated at a low speed, the PWM control is to be carried out in the inverter unit 103 with the battery voltage Vb.
This causes the switching losses of the switching devices (IGBTs and diodes) to become large and, along with this, also on the side of the motor 104, causes ripples in a flowing current to become large. Thus, there is an unsolved problem in that harmonic losses due to carrier frequency components are increased to cause reductions in efficiencies of the load driving system and the motor.
Moreover, when the motor 104 is operated at a high speed, the battery voltage Vb is raised by the bilateral step-up chopper unit 101 as a DC to DC converter circuit. At this time, in the case of the circuit shown in FIG. 16, the IGBT1 and the IGBT2 are operated together with the diodes D1 and D2. Thus, there is also an unsolved problem in that these semiconductor chips are required to have corresponding capacities.
Furthermore, an electric vehicle normally has a battery mounted in which tens of units of batteries each having an output voltage of several volts are connected in series to be provided as a battery with an output voltage of hundreds of volts. However, when only any one unit causes failure for some reason, the battery units connected in series causes the whole of the battery 100 to become unusable. Therefore, in a battery system with battery units simply connected in series as shown in FIG. 15, there is also an unsolved problem in that the reliability of the system can not be improved.
Accordingly, the invention was made by giving attention to the unsolved problems in the above example of a related system with an object of providing a load driving system which can ensure a normal operation of a motor even in the event a failure occurs in a part of a battery while making the load driving system and the motor highly efficient and providing an electric vehicle using the system.